Catalyst and process for preparing amides

ABSTRACT

A platinum complex useful as a catalyst for converting nitriles into amides comprises a platinum complex of dialkyl phosphine of stoichiometric formula: PtX(R 2  POHOPR 2 )(PR 2  OH) were R is an alkyl, alicyclic, chiral, alkylaryl group or substituted alkyl, alicyclic, chiral, alkaryl group or the two R groups attached to one phosphorus atom can form a heterocyclic ring with the phosphorus atom and X is H or a halide. The conversion takes place under reflux conditions to give a high yield of the amide.

This application is a 371 of PCT/GB/00750, filed Mar. 28, 1996.

The present invention relates to platinum phosphorus complexes and their use in a process for the hydration of nitriles to amides.

Amides such as acrylamide, acetamide and benzamide are important industrial chemicals and are used in a wide range of products. Other amides such as nicotinamide are also important as food supplements.

Known methods of making amides from nitriles include the sulphuric acid process in which acrylonitrile is hydrated using sulphuric acid at an elevated temperature and the acrylamide is then separated from the other products by the reaction of cooling.

Another method of preparing amides from nitriles is by the alkaline hydration of nitriles, optionally using a metal catalyst and this process is used to produce nicotinamide. These processes are briefly described in Ullmann's Encyclopaedia of Industrial Chemistry, 5th Edn: Vol. A1, Page 173 and in Kirk Othmer, Encyclopaedia of Chemical Technology, 3rd Edn: Vol 24, Page 67.

However, these processes, unless stringently controlled, can hydrolyse the amide to the acid and this has proved to be a difficulty in commercial operations.

To overcome this problem, enzymes have been used as catalysts, and industrially, heterogenous metal catalysts have been used, particularly to hydrolyse acrylonitrile to acrylamide.

A method of preparing amides and primary alcohols is also described in U.S. Pat. No. 4,684,751 using tertiary trialkyl phosphine complexes of platinum group metals, particularly the trimethyl phosphine complex.

In the process of this invention, nitriles are converted to the corresponding amide using, as a catalyst, a tertiary trimethyl phosphine complex of platinum in the presence of a nucleophile. However, this reaction only works effectively with some nitriles and the reaction needs to be carried out in an oxygen free atmosphere which makes it complex and difficult to operate industrially.

Methods for converting nitriles to amides using noble metal catalysts are also described in C M Jensen and W C Trogler, J Am Chem Soc 1986 108,723(1), G Villain, G Constant, A Gaset and P Kalck, J Mol, Cat. 1980 7,355(2) and T Yoshida, T Matsuda, T Okano, T Kitani and S Otsuka, J Am Chem Soc 1979 101,2027(3).

We have now discovered a class of platinum complexes of dialkyl and diaryl phosphine oxides which selectively convert nitriles to amides.

We have also discovered a method of making platinum complexes of the dialkylphosphine oxides of the invention.

According to the invention there is provided a platinum complex of dialkylphosphine of stoichiometric formula PtX(R₂ POHOPR₂)(PR₂ OH) where R is an alkyl, alicyclic, chiral, alkylarly group or substituted alkyl, alicyclic, chiral, alkaryl group or the two R groups attached to one phosphorus atom can form a heterocyclic ring with the phosphorus atom and X is H or a halide such as chlorine or bromine.

The complexes of the invention are thought to have the structural formula ##STR1## where R and X are as defined above.

In some cases the complex can be in an equilibrium mixture with its hydroxyl form, X═OH.

Preferably R is a C₁ to C₅ alkyl group and more preferably a methyl group.

According to the invention there is also provided a method of hydrating a nitrile to an amide which process comprises reacting the nitrile with water in the presence of the disubstituted phosphinite complex of platinum described above.

The platinum complexes of the present invention can be formed by different methods depending on the nature of X. E.g., when X═H by reacting a platinum triphenyl phosphine complex with a secondary phosphine oxide , R₂ P(O)H, where R and X are as defined above. The reaction is thought to proceed according to the reaction mechanism: ##STR2##

The reaction takes place preferably under normal conditions with regard to temperature and pressure and preferably is carried out in an inert organic solvent such a toluene; preferably the reaction is carried out under an inert atmosphere, e.g. nitrogen.

When X is a halogen by reacting a soluble platinum salt such as K₂ PtCl₄ with a secondary phosphine oxide R₂ P(O)H according to the equation ##STR3##

The reaction proceeds readily and gives a high yield of the desired end product.

By using as a starting material a mixture of substituted phosphinites where the R groups are different, a platinum complex can be obtained having a mixture of substituents.

We have now discovered that the platinum complexes of secondary phosphine oxides of the invention selectively convert nitriles to amides under normal conditions.

The reaction takes place by reacting the nitrile with the catalyst in aqueous solution at room temperature or with heating.

The platinum complex acts as a catalyst and is preferably present in a catalytic amount, e.g. from 0.001 to 1.0 mole per cent based on the weight of the nitrile, preferably 0.01 to 0.1 mole per cent.

The reaction can take place at normal temperature and pressure, although the reaction mixture can be heated to facilitate the reaction. The amount of water used in the reaction mixture is not critical and with the reduced tendency for the amide to hydrolyse to the acid excess water can be used.

For example, the hydrolysis reaction can take place by heating the nitrile with water in the presence of the platinum complex catalyst. If the nitrile is not water soluble an inert solvent such as ethanol or tetrahydrofuran can be added to form a solution.

The compounds where X is a halide, such as chlorine are not strongly active catalysts but reaction with a soluble silver salt, e.g. AgBF₄ converts these into the fully active form.

Alternatively the catalyst can be used as a heterogenous catalyst by supporting the catalyst on a suitable catalyst support. The catalyst suports which can be used include conventional catalyst supports such as silica, alumina, a silico-aluminate, a polymer or copolymer, such as a hydrocarbon polymer or copolymer e.g. a polystyrene polymer or copolymer.

Other phosphinito platinum complexes where X is a different atom or group can be converted into the catalysts of the present invention by replacement of the X moiety.

It is believed that the active catalyst consists of the cationic form and the invention comprises any compound which can give this form in solution. The cationic form is thought to have the structural formula: ##STR4##

The invention can be used to prepare a range of amides, depending on the starting nitrile used, e.g. the invention can be used to convert acrylonitrile to acrylamide, acetonitrile to acetamide, benzonitrile to benzamide, 3-cyanopyridine to nicotinamide, halosubstituted aryl nitrile, e.g. di-fluorophenyl cyanide, to their corresponding amides which can be useful in biological applications such as drugs and biocides. Other nitriles which can be converted to their corresponding amides include polymers and copolymers containing nitrile groups and specific cyanides such as p-hydroxybenzylcyanide which can be converted to the corresponding amide which is sued in the synthesis of Atenolol which is used as a β-blocker.

The invention will now be described in the following examples in which Examples 1-5 are examples of the preparation of the platinum catalyst.

PREPARATION OF CATALYSTS Example 1 Preparation of Pt(H)(PMe₂ OH)(PMe₂ O)₂ H!

Dimethyl phosphine oxide (0.16 g, 2.05 mmol) was added to a stirred solution of tetrakis (triphenyl phosphine) platinum (0) (0.5 g, 0.402 mmol) in dry toluene (10 ml) under nitrogen. Almost instantly a colourless solution was obtained which was stirred for one hour. During this time a white powder separated. Precipitation was completed by addition of ether (20 ml) and stirring for a further hour. The product was filtered off, washed with ether, hexane and dried under suction. Yield: 0.13 g (75%)

The product was analysed and found to have the formula

     Pt(H)(PMe.sub.2 OH)(PMe.sub.2 O).sub.2 H!

Example 2 Preparation of Pt(H)(PPh₂ OH)(PPh₂ O)₂ H!

Diphenyl phosphine oxide (0.45 g, 2.22 mmol) was added to a stirred solution of tetrakis (triphenyl phosphine) platinum (0) (0.55 g, 0.44 mmol) in dry toluene 15 ml) under nitrogen. Almost instantly a pale yellow solution was obtained which was stirred for one hour. During this time a white powder separated. Precipitation was completed by addition of ether (25 ml) and stirring for a further hour. The product was filtered off, washed with ether, hexane and dried under suction. Yield: 0.31 g (88%)

The product was analysed and found to have the formula

    Pt(H)(PPh.sub.2 OH)(PPh.sub.2 O).sub.2 H

Example 3 Preparation of PtCl(PMe₂ OH)(PMe₂ O)₂ H!

Dimethylphosphine oxide (0.17 g 2.18 mmol) in degassed ethanol (3 ml) was added dropwise over two hours to a stirred solution of K₂ PtCl₄ (0.3 g 0.723 mmol) in degassed solution ethanol (9 ml) and H₂ O (5 ml). The reaction mixture was then stirred under nitrogen at 40-50° C. until it turned pale brown. Treatment with charcoal followed by filtration through celite gave a pale yellow solution which was concentrated under reduced pressure. Addition of water gave PtCl(PMe₂ OH)(PMe₂ O)₂ H! (0.2 g, 60%) as fine white needles, which were washed with water, diethyl ether and dried in vacuo.

Example 4 Preparation of PtCl(PMe₂ Ph)(PMe₂ O)₂ H!

A suspension of PtCl(PMe₂ OH(PMe₂ O)₂ H! (0.15 g, 0.324 mmol) in dry CH₂ Cl₂ (20 ml) was treated with dimethylphenylphosphine (0.05 g, 0.362 mmol) and stirred at room temperature under nitrogen for an hour. The clear pale yellow solution was then concentrated under reduced pressure to ca 5 ml. Addition of diethyl ether/60-80 petroleum ether precipitated PtCl(PMe₂ Ph)(PMe₂ O)₂ H! (0.14 g, 83%) as a white powder, which was filtered off, washed with diethyl ether and dried under suction. ##STR5##

Phospholane oxide (0.26 g, 2.50 mmol) in degassed ethanol (3 ml) was added dropwise over two hours to a stirred solution of K₂ PtCl₄ (0.3 g, 0.723 mmol) in degassed ethanol (10 ml) and H₂ O (10 ml). The reaction mixture was then stirred under nitrogen at 40-50° C. for five hours. The resulting pale yellow solution was concentrated under reduced pressure to ca 5 ml. Addition of water gave PtCl(PC₄ H₈ OH)(PC₄ H₈ O)₂ H! (0.22 g, 56%) as a cream powder, which was washed with water, diethyl ether and dried in vacuo.

PREPARATION OF AMIDES Example 6 Preparation of Acetamide

0.0131 g (0.031 mmol) of the platinum complex prepared as Example 1, was stirred vigorously with 7.9 g (0.19 mol) of acetonitrile and 5 ml (0.28 mol) of water in a flask.

The resulting mixture was heated at 100° C. for 15 hours under reflux and allowed to cool. The cooled mixture was concentrated by evaporation under reduced pressure. A white powder was precipitated and was filtered from the liquid, dried in vacuo and weighed to give a yield of 10.2 grams. Analysis of this precipitate showed it to be acetamide and the yield was calculated at 90% on the weight of acetonitrile.

Example 7 Preparation of Acrylamide

0.0096 g (0.022 mmol) of the catalyst of Example 1 was stirred vigorously with 8.1 g (0.15 mol) of acrylonitrile, 6 ml (0.33 mol) of water and 25 ml of ethanol in a flask and heated at 110° C. for 15 hours under reflux. After cooling the liquids were removed using a rotary evaporator to give 10.12 grams of a white powder which, on analysis, was found to be acrylamide; the yield was calculated to be 94% based on the weight of acrylonitrile.

Example 8 Preparation of Acrylamide

PtCl(PMe₂ OH)(PMe₂ O)₂ H! made as in Example 4 (0.0052 g, 0.0112 mmol) was dissolved in a vigorously stirred mixture of acrylamide (4.0 g, 0.076 mol), ethanol (4 ml) and water (3 ml, 0.166 mol). To this was added an excess of AgBF₄ (0.0062 g, 0.019 mmol) and the reaction mixture refluxed at 90° C. for three hours. After cooling, it was filtered to remove the precipitated AgCl. The solvent was then removed under reduced pressure to give acrylamide (4.50 g, 83%) which was dried in vacuo.

Example 9 Preparation of Acetamide

(0.010 g, 0.0216 mmol) of the catalyst prepared as in Example 3, was dissolved in a vigorously stirred mixture of acetonitrile (3.9 g, 0.096 mol) and water (3.5 ml, 0.194 mol). To this was added an excess of AgBF₄ (0.0057 g, 0.34 mmol) and the reaction mixture refluxed for three hours. After cooling, it was filtered to remove the precipitated AgCl. The solvent was then removed under reduced pressure to give acetamide (4.35 g, 77%), which was dried in vacuo.

Example 10 Preparation of Acrylamide

(0.0052 g, 0.0112 mmol) of the catalyst prepared in Example 3, was dissolved in a vigorously stirred mixture of acrylonitrile (4.0 g, 0.076 mol), ethanol (4 ml) and water (3 ml, 0.166 mol). To this was added an excess of AgBF₄ (0.0062 g, 0.019 mmol) and the reaction mixture refluxed for three hours. After cooling, it was filtered to remove the precipitated AgCl. The solvent was then removed under reduced pressure to give acrylamide (4.50 g, 83%), which was dried in vacuo.

Example 11 Preparation of Benzamide

(0.0045 g, 0.0097 mmol) of the catalyst of Example 3, was dissolved in a vigorously stirred mixture of benzonitrile (3.5 g, 0.034 mmol), ethanol (7 cm³) and water (2 ml, 0.06 mmol). To this was added an excess of AgBF₄ (0.0045 g, 0.019 mmol) and the reaction mixture refluxed at 90° C. for five hours. After cooling, it was filtered to remove the precipitated AgCl. The solvent was then removed under reduced pressure to give benzamide (3.60 g, 87%), which was dried in vacuo.

                                      TABLE 1     __________________________________________________________________________     Catalytic Hydration of Various Nitriles using     (PtH(PMe.sub.2 OH)(PMe.sub.2 O).sub.2)                                    Turnover Rate                                             Isolated     Nitrile    Product    Solvent  mol/mol of cat.h)                                             Yield %     __________________________________________________________________________     H.sub.2 C═CHCN                H.sub.2 C═CHC)NH.sub.2                           aqueous ethanol                                    1485     93     NC(CH.sub.2).sub.4 CN                H.sub.2 NCO(CH.sub.2).sub.4 CONH.sub.2                           aqueous THF                                     55      97     1 #STR6##                6 #STR7##  aqueous ethanol                                    518      86     2 #STR8##                7 #STR9##  water    450      91     3 #STR10##                8 #STR11## aqueous THF                                    220      73     4 #STR12##                9 #STR13## aqueous THF                                    123      92     5 #STR14##                0 #STR15## aqueous THF                                    173      96     __________________________________________________________________________      *Rate at which the diamide is produced

Example 12 Preparation of Pyridine 4-Carboxamide

(0.0051 g, 0.0119 mmol) of the platinum complex, prepared as in Example 1, was stirred with 4-cyanopyridine (2.0 g, 0.019 mol) and 0.75 ml (0.042 mol) of water and 10 ml tetrahydrofuran and the mixture heated under reflux for 4 hours. After cooling the liquids were removed using a rotary evaporator to give pyridine 4-carboxamide (2.12 g) corresponding to 90% yield.

Example 13 Preparation of 4-Hydroxy Phenylacetamide

(0.0032 g, 0.007 mmol) of the platinum complex, prepared as in Example 1, was stirred with 4-hydroxybenzyl cyanide (1.0 g, 7.5 mmol) and 5 ml (0.28 mol) of water and 5 ml ethanol and the mixture heated under reflux for 18 hours. After cooling the liquids were removed using a rotary evaporator to give 4-hydroxy phenylacetamide (1.116 g) corresponding to 98% yield.

Examples 14 to 18

Various nitriles were hydrated to the corresponding amides using the process of Example 6 and the results shown in Table 1.

Example 19

A comparison was made with other catalysts for the hydration of acetonitrile to acetamide and the results shown in the following Table 2.

                                      TABLE 2     __________________________________________________________________________     Comparison of the Catalytic Activities for the Hydration of Acetonitrile     to     Acetamide                      Turnover  Turnover                  Temp.                      Frequency Number     Catalyst     (° C.)                      mol/(mol of cat. h)                                mol/(mol of cat)                                        Ref     __________________________________________________________________________      PtH(PMe.sub.2 OH)(PMe.sub.2 O).sub.2 H!                  90  380       5,700      PtH(H.sub.2 O)(PMe.sub.3).sub.2 ! OH!                  78  178       5,000-6,000                                        (1)      PtH(H.sub.2 O)(PEt.sub.3).sub.2 ! OH!                  78  70        not reported                                        (1)     PdCl(OH)(bipy)(H.sub.2 O)                  76  29        not reported                                        (2)     Pt P(cC.sub.6 H.sub.11).sub.3 !.sub.2                  80  27        405     (3)     Pt(PEt.sub.3).sub.3                  80  3         54      (3)     NaOH         78  0.4     __________________________________________________________________________

The references refer to those on Page 1 of the specification. In the case of acrylonitrile, the turnover number was in excess of 50,000.

The selectivity of the catalyst of the invention is shown in Table 3 where various catalysts were used in the hydration of acrylonitrile and the amount of the unwanted alternative products. β-cyanoethanol and β,β dicyano ethyl ether produced by hydration of the olefinic double bond in acrylonitrile determined . The superiority of the catalyst of the invention is the turnover frequency and/or selectivity is clearly seen.

                                      TABLE 3     __________________________________________________________________________     Catalytic Hydration of Acrylonitrile                 Temp.                     Turnover frequency: mol/(mol of cat. h)                                            Selectivity for                 (° C.)                     acrylamide                           β-cyanoethanol                                  β,β-dicyanoethylether                                            Nitrile (%)     __________________________________________________________________________     (PtH(PMe.sub.2 OH)(Me.sub.2 O).sub.2 H)                 90  1485  /      /         >99     (PtH(H.sub.2 O)(PMe.sub.3).sub.2 (OH)                 25  6.2   0.02   0.19      97     (PtH(H.sub.2 O)(PMe.sub.3).sub.2 (OH)                 80  65    84.5   10.5      29     Pt(PPr.sup.1.sub.3).sub.3                 80  1.8   2.5    20.9      7.5     (Pt(NHCOMe)(Ph)(PEt.sub.3).sub.2)                 80  2.2   0.25   2.45      45     __________________________________________________________________________ 

We claim:
 1. A platinum complex of dialkylphosphine of stiochiometric formula PtX(R₂ POHOPR₂)(PR₂ OH) where R is a C₁ to C₅ alkyl group and X is H or a halide.
 2. A platinum complex as claimed in claim 1 in which R is a methyl group.
 3. A platinum complex as claimed in claim 1 in which X is bromine or chlorine.
 4. A platinum complex as claimed in claim 2 in which X is bromine or chlorine.
 5. A platinum complex of dialkylphosphine which in solution forms a cation of the structural formula: ##STR16## where R is C₁ to C₅ alkyl group.
 6. A method of forming a platinum complex of dialkylphosphine of stoichiometric formula PtH(R₂ POHOPR₂)(PR₂ OH) where R is a C₁ to C₅ alkyl group and X is H or a halide which method comprises reacting a platinum triphenyl phosphine complex of formula (R¹ ₃)P₄ with a secondary phosphine oxide of formula R₂ P(O)H, where R is as defined above and R¹ is a phenyl or substituted phenyl group.
 7. A method of forming a platinum complex of dialkylphosphine of stoichiometric formula PtX(R₂ POHOPR₂) (PR₂ OH) where R is a C₁ to C₅ alkyl group and X is a halogen by reacting a platinum salt with a secondary phosphine oxide of formula R₂ P(O)H.
 8. A method of forming a platinum complex as claimed in claim 7 in which the platinum salt is K₂ PtCl₄.
 9. A method as claimed in claim 7 in which the reaction is carried out in an inert organic solvent.
 10. A method as claimed in claim 8 in which the reaction is carried out in an inert organic solvent.
 11. A platinum complex formed by the method of claim
 6. 12. A platinum complex formed by the method of claim
 7. 13. A platinum complex formed by the method of claim
 9. 14. A platinum complex formed by the method of claim
 10. 15. A platinum complex formed by reacting a platinum complex as claimed in claim 1 where X is a halide, with a soluble silver salt.
 16. A platinum complex as claimed in claim 15 in which the soluble silver salt is AgBF₄.
 17. A catalyst composition which comprises a platinum complex as claimed in claim 1 on an inert catalyst support.
 18. A catalyst composition which comprises a platinum complex as claimed in claim 5 on an inert catalyst support.
 19. A catalyst composition which comprises a platinum complex as claimed in claim 13 on an inert catalyst support.
 20. A catalyst composition which comprises a platinum complex as claimed in claim 15 on an inert catalyst support.
 21. A catalyst composition as claimed in claim 17 in which the catalyst support comprises silica, alumina, a silico-aluminate, a polymer or copolymer or a polystyrene polymer or copolymer.
 22. A catalyst composition as claimed in claim 18 in which the catalyst support comprises a silica, alumina, a silico-aluminate, a polymer or copolymer or a polystyrene polymer or copolymer.
 23. A catalyst composition as claimed in claim 19 in which the catalyst support comprises silica, alumina, a silico-aluminate, a polymer or copolymer or a polystyrene polymer or copolymer.
 24. A catalyst composition as claimed in claim 20 in which the catalyst support comprises silica, alumina, a silico-aluminate, a polymer or copolymer or a polystyrene polymer or copolymer. 